A measuring pen calibration tool and calibration method for industrial photogrammetry

Abstract

The present invention provides a measuring pen calibration tool and calibration method for industrial photogrammetry, which belongs to the technical field of measuring pen calibration. The calibration tool of the present invention comprises a tool body, which is in a cubic shape. The top of the tool body is provided with calibration holes of several specifications to adapt to measuring pens with different probe diameters. The coordinates of the probe placed in the calibration hole coincide with the center coordinates of the calibration hole. Coding points are provided on the four sides of the tool body. The calibration method of the present invention uses the calibration tool of the present invention. Compared with the traditional method, the present invention has a simple calibration process, and only one measurement is required to complete the calibration of the measuring pen; the entire calibration process has low environmental requirements; for measuring rods of different lengths, the calibration results are consistent and reliable.

Description

Technical Field

[0001] The invention relates to the technical field of measuring pen calibration, and in particular to a measuring pen calibration tool and a calibration method for industrial photogrammetry. Background Art

[0002] The measuring pen is one of the standard configurations of most industrial photogrammetry systems. It can be used to perform contact measurement of the hidden geometric features of the workpiece. The measuring pen is generally composed of the measuring pen main structure, control points, probe rod and probe (the probe is spherical).

[0003] The design coordinates of the control points of the measuring pen are generally accurately calibrated and known before leaving the factory. Before using the measuring pen, it is only necessary to calibrate the coordinate values ​​of the measuring pen probe point in the design coordinate system of the control point. This calibration process is generally called measuring pen calibration. After the calibration of the measuring pen is completed, the photogrammetry system can measure the coordinate values ​​of the measuring pen control points, and then calculate the conversion parameters from the design coordinates of the measuring pen control points to the measurement coordinate system of the photogrammetry system by combining the design values ​​of the control points through the least squares spatial similarity transformation, and thus convert the probe point coordinates to the measurement coordinate system of the photogrammetry system to obtain the coordinate values ​​of the probe point in the measurement coordinate system of the photogrammetry system. It can be seen that the calibration accuracy of the measuring pen will directly affect its measurement accuracy.

[0004] The current calibration method for measuring pens is to place the measuring pen head into a stable conical hole, rotate the measuring pen at different angles, and simultaneously obtain the corresponding coordinates of the measuring pen control points. Ideally, the moving trajectory of each measuring pen control point is on a sphere with the measuring pen head as the center. Therefore, the coordinates of the measuring pen head can be obtained by fitting the center of the sphere using the least squares method with the coordinates of the control points at different angles as input, and the calibration of the measuring pen can be completed. The disadvantages of this calibration method are as follows:

[0005] 1) The calibration process is complicated and requires multiple measurements to complete the calibration;

[0006] 2) The calibration process requires a stable environment. If the tapered hole used for calibration is displaced and deformed during the calibration process, it will directly affect the final calibration accuracy. Therefore, this calibration method has poor environmental adaptability.

[0007] 3) For measuring pens with too long measuring rods, the multi-angle rotation process is likely to cause deformation of the measuring rod, thereby affecting the calibration accuracy.

[0008] Therefore, it is necessary to provide a measuring pen calibration tool and a calibration method for industrial photogrammetry to solve the above-mentioned existing problems. Summary of the invention

[0009] In view of this, the present invention provides a measuring pen calibration tool and a calibration method for industrial photogrammetry to solve the problems existing in the background technology.

[0010] In order to solve the above technical problems, the present invention provides a measuring pen calibration tool for industrial photogrammetry, including a tool body, which is in a cubic shape. The top of the tool body is provided with calibration holes of several specifications to adapt to measuring pens with different probe diameters. The coordinates of the probe placed in the calibration hole coincide with the center coordinates of the calibration hole. Coding points are provided on the four side walls of the tool body.

[0011] Furthermore, the measuring pen comprises a main body structure, a plurality of control points are arranged on the main body structure, a measuring rod is arranged at the bottom of the main body structure along the length direction of the main body structure, and a measuring head is arranged at the bottom of the measuring rod.

[0012] Furthermore, the probe is spherical.

[0013] A method for calibrating a measuring pen for industrial photogrammetry uses the above-mentioned measuring pen calibration tool for industrial photogrammetry.

[0014] Further, the following steps are included:

[0015] Step 1: Place the probe in the appropriate calibration hole, and directly measure the coordinates of each coding point and control point in the photogrammetric coordinate system through the photogrammetric system;

[0016] Step 2: The measured coordinates of the coding point are combined with its calibration coordinates to perform a least squares spatial similarity transformation to obtain a transformation relationship from the calibration coordinate system of the coding point to the measured coordinate system;

[0017] Step 3: Calculate the measurement coordinate value of the adapted calibration hole, which is the measurement coordinate value of the probe;

[0018] Step 4: Perform the least squares spatial similarity transformation based on the coordinate design value and the measured value of the control point to obtain the transformation relationship from the measurement coordinate system to the design coordinate system. The measurement coordinates of the probe are converted to the design coordinate system of the control point to complete the calibration.

[0019] Furthermore, the least squares spatial similarity transformation is the transformation of a rigid body from one coordinate system to another coordinate system. The relative relationship between the two coordinate systems can be any translation, rotation or overall scaling.

[0020] Furthermore, the spatial similarity transformation is also called 3D Helmert transformation, which has three translations [X s Y s Z s], 3 rotation quantities [RxRyRz] and a scaling factor m, a total of 7 parameters to establish the transformation model, among which, the 3 rotation quantities can form a rotation matrix R. Suppose the coordinates of a coordinate point (XYZ) transformed to the new coordinate system are (X′Y′Z′), then:

[0021]

[0022] The above technical solution of the present invention includes at least the following beneficial effects:

[0023] 1. Compared with the traditional method, the calibration process is simple, and only one measurement is required to complete the calibration of the measuring pen;

[0024] 2. The entire calibration process has low environmental requirements;

[0025] 3. The calibration results for probe rods of different lengths are consistent and reliable. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 A three-dimensional diagram of the calibration tooling of the present invention;

[0027] Figure 2 It is a structural schematic diagram of a measuring pen in the prior art.

[0028] In the figure:

[0029] 1. Calibration surface; 2. Calibration hole; 3. Marking surface; 4. Coding point; 5. Control point; 6. Main structure; 7. Measuring rod; 8. Measuring head. DETAILED DESCRIPTION

[0030] To make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the following will be combined with the appended drawings of the embodiments of the present invention. Figure 1-2 , the technical scheme of the embodiment of the present invention is clearly and completely described. Obviously, the described embodiment is a part of the embodiment of the present invention, not all of the embodiments. Based on the described embodiment of the present invention, all other embodiments obtained by ordinary technicians in this field belong to the scope of protection of the present invention.

[0031] Example

[0032] A measuring pen calibration tool for industrial photogrammetry includes a tool body, which is in a cubic shape. The top of the tool body is provided with calibration holes 2 of several specifications to adapt to measuring pens with different probe 8 diameters. The coordinates of the probe 8 placed in the calibration hole 2 coincide with the center coordinates of the calibration hole 2. Coding points 4 are provided on the four side walls of the tool body.

[0033] Specifically, a measuring pen calibration tool for industrial photogrammetry, such as Figure 2As shown, the measuring pen is a prior art, and comprises a main structure 6, a measuring rod 7 and a measuring head 8, and a control point 5 is provided on the main structure 6. Figure 1 As shown, the calibration tool includes a tool body, which is in a cubic shape. The top of the tool body is a calibration surface 1, and calibration holes 2 of several specifications are opened on the calibration surface 1 to adapt to measuring pens with different diameters of probes 8. The coordinates of the probe 8 placed in the calibration hole 2 coincide with the center coordinates of the calibration hole 2. The side walls around the tool body are four marking surfaces 3, and each marking surface 3 is provided with several coding points 4. The positional relationship between the coordinates of each coding point 4 and each calibration hole 2 can be obtained by precise calibration.

[0034] A method for calibrating a measuring pen for industrial photogrammetry, using the above-mentioned industrial photogrammetry measuring pen calibration tool, specifically comprises the following steps:

[0035] Step 1: Place the probe 8 in the adapted calibration hole 2, and directly measure the coordinates of each encoding point 4 and control point 5 in the photogrammetric coordinate system through the photogrammetric system;

[0036] Step 2: Perform a least squares spatial similarity transformation on the measured coordinates of the coding point 4 in combination with its calibration coordinates to obtain a transformation relationship from the calibration coordinate system of the coding point 4 to the measured coordinate system;

[0037] Step 3: Calculate and obtain the measurement coordinate value of the adapted calibration hole 2, which is the measurement coordinate value of the probe 8;

[0038] Step 4: Perform the least squares spatial similarity transformation based on the coordinate design value and the measured value of the control point 5 to obtain the transformation relationship from the measured coordinate system to the designed coordinate system, thereby converting the measured coordinates of the probe 8 to the design coordinate system of the control point 5 to complete the calibration.

[0039] The least squares spatial similarity transformation is the transformation of a rigid body from one coordinate system to another. The relative relationship between the two coordinate systems can be any translation, rotation or overall scaling.

[0040] Spatial similarity transformation is also called 3D Helmert transformation, which has three translations [X s Y s Z s ], 3 rotation quantities [RxRyRz] and a scaling factor m, a total of 7 parameters are used to establish the transformation model. Among them, the 3 rotation quantities can form a rotation matrix R.

[0041] Suppose the coordinates of a coordinate point (XYZ) converted to a new coordinate system are (X′Y′Z′), then:

[0042]

[0043] The point in another coordinate system corresponding to a coordinate point is called a common point. Therefore, at least 3 groups of common points can solve 7 transformation parameters. In practical applications, in order to ensure the reliability of the solution, at least 4 groups of common points are required to perform least squares adjustment to obtain 7 transformation parameters.

[0044] The target coordinate system of the general conversion is called the design coordinate system or reference coordinate system, and the other coordinate system is called the measurement coordinate system. Calculate the seven transformation parameters from the measurement coordinate system to the design coordinate system, of which three translations are the translations of the measurement coordinate system relative to the design coordinate system, and three rotations are the postures of the measurement coordinate system relative to the design coordinate system.

[0045] Test example

[0046] In order to verify the effectiveness of the calibration tooling and calibration method proposed in the present invention, an experimental comparison was conducted with the traditional method.

[0047] The experiment is mainly divided into two parts:

[0048] 1) Select a measuring pen (model: MPS-PEN / 40 / 050) with a measuring rod length of 50 mm, and use the calibration tool and calibration method of the present invention and the traditional equipment and method for comparison;

[0049] 2) Select a measuring pen (model: MPS-PEN / 40 / 150) with a measuring rod length of 150 mm, and use the calibration tool and calibration method of the present invention and the traditional equipment and method for comparison.

[0050] The comparison indicators are as follows:

[0051] 1) The entire calibration process is time-consuming;

[0052] 2) Calibration accuracy: The calibration accuracy of the measuring pen is verified by measuring the length deviation of a standard 400mm long gauge block with the calibrated measuring pen.

[0053] Table 1. Experimental results of 50 mm probe rod

[0054]

[0055] Table 2. Experimental results of 150 mm probe rod

[0056]

[0057] It can be seen from the experimental results in Table 1 and Table 2 that the calibration tooling and calibration method proposed in the present invention have the same calibration accuracy as the traditional method when the measuring rod is relatively short, but the time consumption is significantly less; when the measuring rod is relatively long, the calibration tooling and method proposed in the present invention have significantly better accuracy than the traditional method, and are consistent with the calibration accuracy of the short measuring rod measuring pen.

[0058] In the present invention, unless otherwise clearly stipulated and limited, for example, it can be a fixed connection, a detachable connection, or an integrated one; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. Unless otherwise clearly defined, for ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to the specific circumstances.

[0059] The above are preferred embodiments of the present invention. It should be pointed out that, for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for calibrating a measuring pen for industrial photogrammetry, characterized in that: The method is realized by means of a measuring pen calibration tool for industrial photogrammetry, the tool comprising a tool body, the tool body is in a cubic shape, a plurality of calibration holes of different specifications are arranged on the top of the tool body to adapt to measuring pens with different probe diameters, the coordinates of the probe put into the calibration hole coincide with the center coordinates of the calibration hole, and coding points are arranged on the four sides of the tool body; the measuring pen comprises a main structure, a plurality of control points are arranged on the main structure, a measuring rod is arranged at the bottom of the main structure along the length direction of the main structure, and a measuring head is arranged at the bottom of the measuring rod; The calibration method of the measuring pen for industrial photogrammetry includes the following steps: Step 1: Place the probe in the appropriate calibration hole, and directly measure the coordinates of each coding point and control point in the photogrammetric coordinate system through the photogrammetric system; Step 2: The measured coordinates of the coding point are combined with its calibration coordinates to perform a least squares spatial similarity transformation to obtain a transformation relationship from the calibration coordinate system of the coding point to the measured coordinate system; Step 3: Calculate the measurement coordinate value of the adapted calibration hole, which is the measurement coordinate value of the probe; Step 4: Perform the least squares spatial similarity transformation based on the coordinate design value and the measured value of the control point to obtain the transformation relationship from the measurement coordinate system to the design coordinate system, thereby converting the measurement coordinates of the probe to the design coordinate system of the control point to complete the calibration; The least squares spatial similarity transformation is the transformation of a rigid body from one coordinate system to another. The relative relationship between the two coordinate systems is an arbitrary translation, rotation, or overall scaling. The least squares spatial similarity transformation is also called 3D Helmert transformation, which has three translations [X s Y s Z s ], 3 rotation quantities [RxRyRz] and a scaling factor m, a total of 7 parameters to establish the transformation model, among which the 3 rotation quantities constitute the rotation matrix R. Suppose the coordinates of a coordinate point (XYZ) transformed to the new coordinate system are (X'Y'Z'), then:

2. The method for calibrating a measuring pen for industrial photogrammetry according to claim 1, characterized in that: The measuring head is spherical.

Images